This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-181727, filed Jun. 16, 2000; and No. 2000-192802, filed Jun. 27, 2000, the entire contents of both of which are incorporated herein by reference.
The present invention relates to a manufacturing method and a manufacturing apparatus for a metallic bellows incorporated in an accumulator, vacuum valve, or pump.
A metallic bellows has top portions and bottom portions that are formed alternately in its axial direction. The top and bottom portions constitute pleat walls. Conventionally, the top and bottom portions have V-, xcexa9-, or S-shaped profiles, besides U-shaped profiles. When a bellows that has pleat walls with S-shaped profiles is compressed in its axial direction, its length (compact-state length) is shorter enough than that of an ordinary bellows with U-shaped pleat walls. Thus, the S-profiled bellows can enjoy a long stroke for extension and contraction from its free length. The xe2x80x9cfree lengthxe2x80x9d described herein is the axial length of the bellows that is not subjected to any external force. The xe2x80x9ccompact-state lengthxe2x80x9d is the axial length of bellows obtained when the bellows is compressed so that pleat walls come into contact with one another.
In a known method for manufacturing an S-profiled metallic bellows, a plurality of disc-shaped bellows elements with S-shaped profiles that are formed by pressing, for example, are welded in succession to one another. In an alternative method, top portions and bottom portions are formed integrally with one another by bulging a metallic blank tube as a material of a bellows. The former is called a welded bellows, and the latter a formed bellows. The formed bellows has an advantage over the welded bellows in enjoying higher yield of material, small number of manufacturing steps and steadier quality.
As an example of bulging, hydraulic forming may be used integrally to form an S-profiled bellows. In the bellows formed by the hydraulic forming, the radius of curvature of the distal end of each bottom portion, in particular, is considerably greater than that of the distal end of each top portion. Accordingly, the hydraulic forming only cannot make the most of the advantage (shorter compact-state length) of the S-profiled bellows.
A bellows manufacturing apparatus for hydraulic forming comprises first and second dies that are arranged around a blank tube as a material of a bellows. Hydraulic pressure is applied from inside the tube to expand a part of the tube between the first and second dies. At the same time, these dies are moved toward each other so that the expanded part of the tube is held between them, whereupon pleat walls are formed.
The bellows manufacturing apparatus of this type has a problem that if the taper angles of the respective opposite forming surfaces of the paired dies are narrow, the pleat walls are scratched as the dies are opened in the diametrical direction of the tube after the walls are formed. The pleat walls can be prevented from being scratched by widening the taper angles of the forming surfaces of the dies. If the taper angles of the forming surfaces are wide, however, the distance between the respective distal end portions of the dies is so long that the pleat walls cannot be easily formed into desired corrugated configurations (S-shaped configurations).
Accordingly, a first object of the present invention is to provide a manufacturing method for a metallic bellows, whereby the compact-state length of a metallic bellows having pleat walls with S-shaped profiles can be made short enough and the elastic stroke of the bellows can be adjusted to a desired value.
A second object of the invention is to provide a bellows manufacturing apparatus capable of forming pleat walls in accurate shapes without scratching a bellows with S-shaped profiles.
A manufacturing method for a metallic bellows of the present invention that achieves the first object comprises: a primary forming process for forming top portions and bottom portions, having pleat walls with S-shaped profiles, on a metallic blank tube as a material of the bellows so as to be arranged alternately in the axial direction of the tube, thereby obtaining a formed bellows; a pressing process for compressing the formed bellows in the axial direction; and a stretching process for pulling the bellows in the axial direction, thereby obtaining desired pitches and free length, after the pressing process.
According to the bellows manufacturing method of the invention, the compact-state length of the bellows having the S-profiled pleat walls can be made short enough, and a metallic bellows having a desired elastic stroke can be manufactured.
In this bellows manufacturing method, the stretching process may be followed by an annealing and ageing heat-treatment process (removal of distortion), which is carried out at a temperature of, for example, 400xc2x0 C. to 600xc2x0 C., in order to increase a spring limit value. According to this invention, the annealing and ageing heat-treatment process improves the elastic limit of the bellows as a spring that extends and contracts repeatedly, and therefore, the durability of the bellows.
In the manufacturing method of the invention, moreover, the heat-treatment process may be followed by a setting process to improve permanent set of the bellows and obtain desired pitches and free length. According to this invention, the pitches and length of the bellows can be adjusted, and the permanent set of the bellows can be improved.
In the manufacturing method of the invention, furthermore, the pressing process may include applying an axial load to the formed bellows and applying hydraulic pressure to the bellows from inside, thereby reducing the radius of curvature of a distal end of each bottom portion of the bellows. According to this invention, the compact-state length of the bellows can be reduced.
A bellows manufacturing apparatus of the invention that achieves the second object comprises: a first die provided around a blank tube; a second die located at a distance from the first die in the axial direction of the tube and dividable in the diametrical direction of the tube; first seal means provided on the inner surface of the tube so as to be located corresponding to the first die; second seal means provided on the inner surface of the tube so as to be located corresponding to the second die and defining a hydraulic chamber in conjunction with the first seal means; hydraulic supply means for supplying a compressed liquid to the hydraulic chamber, thereby causing a part of the tube to expand outward; a die drive mechanism for moving the second die toward the first die, thereby plastically deforming the expanded region of the tube to form pleat walls between the first die and the second die; fine-retreat means for slightly retreating the second die away from the pleat walls before the second die is opened in the diametrical direction of the tube after the pleat walls are formed; a die opening/closing mechanism for opening the second die in the diametrical direction after the second die is retreated by means of the fine-retreat means; and a tube feed mechanism for relatively moving the tube for a given distance in the axial direction of the tube with respect to the second die and the first die after the second die is opened in the diametrical direction.
In the bellows manufacturing apparatus of the invention, a part of the blank tube expands outward as the pressurized liquid is supplied to the hydraulic chamber between the first and second seal means. At the same time, the second die moves toward the first die. Thus, the expanded part of the tube is elastically deformed between the first and second dies, whereupon the pleat walls are formed. After the pleat walls are formed, the fine-retreat means slightly retreats the second die away from the pleat walls. Thereafter, the second die opens in the diametrical direction. After the second die is opened in the diametrical direction, the tube feed mechanism causes the tube to move for the given distance in the axial direction with respect to the second die and the first die. At the same time, the second die is retreated to its initial position.
According to the bellows manufacturing apparatus of the invention, the bellows can be formed having desired pleat walls with S-, V-, xcexa9-, or U-shaped profiles, depending on the shapes of forming surfaces of the dies. In the case where the pleat walls of bellows are formed by means of irregular forming surfaces with, for example, S-shaped profiles, in particular, they can be prevented from being scratched by the forming surfaces as the dies open in the diametrical direction even if the respective taper angles of the forming surfaces are narrow.
In the bellows manufacturing apparatus of the invention, the respective opposite surfaces of the first die and the second die are provided individually with forming surfaces for forming the pleat walls with S-shaped profiles, for example.
In the bellows manufacturing apparatus of the invention, the respective taper angles of the forming surfaces, with respect to segments perpendicular to the axis of the tube, should be narrow angles of 10xc2x0 or less. According to this invention, the distance between the respective distal end portions of first die and the second die during forming operation can be made shorter. Thus, the shape of the pleat walls with the S-shaped profiles is stabilized, so that a high-durability metallic bellows can be manufactured.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.